A tunnel field effect transistor (TFET) is a transistor which has different types of source and drain semiconductors, which has asymmetric diffusion layers and to which an LSI of little power consumption is applicable. A p-n-i-n type TFET is known in which a pocket region which has conductivity opposite to a source is inserted on a source side.
However, there is a problem that, in a conventional p-n-i-n type TFET, an inversion layer and a pocket region are joined, thereby increasing junction leakage and deteriorating sub-threshold swing (referred to as “SS” below). Further, a mode of MOSFET deriving from a parasitic n-i-n type structure appears, and therefore there is a problem that steep SS cannot be realized.